The present invention relates to helically corrugated tubing having corrugations of variable depth and the method of manufacturing such tubing.
The manufacture of helically corrugated tubing is old in the art as indicated by U.S. Pat. Nos. 3,015,355 and 3,553,267. In general, a plain-walled tube having a cylindrical or other cross section is locally stressed at a plurality of points in a common plane to weaken the tube and start the formation of corrugations. The tube is installed in a twisting machine such as a lathe with one end of the tube engaged by a rotatable chuck in the headstock and the opposite end restrained against rotation by a non-rotatable chuck in the tailstock. A mandrel is located coaxially within the tube and the rotatable chuck is then turned to twist the one end of the tube relative to the other while at the same time axially directed forces are applied to the tube by pushing the tailstock toward the headstock. Deformation of the tube wall progresses along a spiral or helical path to form the corrugations and the depth of such corrugations is limited by the mandrel within the tube. As indicated in U.S. Pat. No. 3,533,267, the rate of rotation of the tube relative to the rate of axial movement of the tailstock toward the headstock controls the shape and pitch of the corrugations.
The corrugations in tubes facilitate their use in many different areas particularly in the heat exchange field where one fluid passes within the tube in heat exchange relationship with another fluid on the outside of the tube. The corrugations in the tube wall increase the surface area of the tube per unit of tube length and also create turbulent flow inside and outside of the tube to improve heat transfer coefficients at the inner and outer tube surfaces.
Tubing units incorporating helically corrugated tubing can be formed by composites of both plain-walled and corrugated tubing as indicated in U.S. Pat. No. 3,730,229. In addition, shell-and-tube heat exchangers incorporate spiral tubing in tube bundles in order to gain the benefit of improved heat transfer coefficients in the bundle design.
In the prior art heat exhangers, the corrugations in the tubing are generally uniform from one end of the tube to the other and, correspondingly, the pitch and shape of the corrugations remain substantially the same from one end of the tube to the other. While such corrugations improve the heat transfer coefficients by virtue of the larger surface areas and induced turbulence, the cross sectional area of the tube remains unchanged as in a conventional tube and any changes in state or density of the fluid mediums are not accommodated. Increased pressure levels or velocities and backpressure are experienced.
It is a general object of the invention to disclose a new and novel tubing having a tube wall formed with helical corrugations that vary in depth along the length of the tubing and effectively change the cross sectional flow area through or over the tubing. It is also an object of the present invention to disclose a method for making the novel tubing. It is a further object of the invention to provide a heat exchanger utilizing the novel tubing.